What we have learned about the process of brain development helps us understand more about the roles both genetics and the environment play in our development. It appears that genetics predispose us to develop in certain ways, but our experiences, including our interactions with other people, have a significant impact on how our predispositions are expressed. Research now shows that many capacities thought to be fixed at birth are actually dependent on a sequence of experiences combined with heredity. Both factors are essential for optimum development of the human brain (Shonkoff & Phillips, 2000).
Early brain development
The raw material of the brain is the nerve cell, called the neuron. During fetal development, neurons are created and migrate to form the various parts of the brain. As neurons migrate, they also differentiate, or specialise, to govern specific functions in the body in response to chemical signals (Perry, 2002). This process of development occurs sequentially from the ‘bottom up,’ that is, from areas of the brain controlling the most primitive functions of the body (e.g., heart rate, breathing) to the most sophisticated functions (e.g., complex thought) (Perry, 2000). The first areas of the brain to fully develop are the brainstem and midbrain; they govern the bodily functions necessary for life, called the autonomic functions. At birth, these lower portions of the nervous system are very well developed, whereas the higher regions (the limbic system and cerebral cortex) are still rather primitive. Higher function brain regions involved in regulating emotions, language, and abstract thought grow rapidly in the first three years of life (Zero to Three, 2012).
Brain development in childhood
Brain development or learning, is actually the process of creating, strengthening, and discarding connections among the neurons. These connections are called synapses. Synapses organise the brain by forming pathways that connect the parts of the brain governing everything we do—from breathing and sleeping to thinking and feeling. This is the essence of postnatal brain development, because at birth, very few synapses have been formed. The synapses present at birth are primarily those that govern our bodily functions such as heart rate, breathing, eating, and sleeping. The development of synapses occurs at an astounding rate during a child’s early years in response to that child’s experiences. At its peak, the cerebral cortex of a healthy toddler may create two million synapses per second (Zero to Three, 2012). By the time children are two years old, their brains have approximately 100 trillion synapses, many more than they will ever need. Based on the child’s experiences, some synapses are strengthened and remain intact, but many are gradually discarded. This process of synapse elimination—or pruning—is a normal part of development (Shonkoff & Phillips, 2000). By the time children reach adolescence, about half of their synapses have been discarded, leaving the number they will have for most of the rest of their lives.
Another important process that takes place in the developing brain is myelination. Myelin is the white fatty tissue that forms a sheath to insulate mature brain cells, thus ensuring clear transmission of neurotransmitters across synapses. Young children process information slowly because their brain cells lack the myelin necessary for fast, clear nerve impulse transmission (Zero to Three, 2012). Like other neuronal growth processes, myelination begins in the primary motor and sensory areas (the brain-stem and cortex) and gradually progresses to the higher-order regions that control thought, memories, and feelings. Also, like other neuronal growth processes, a child’s experiences affect the rate and growth of myelination, which continues into young adulthood (Shonkoff & Phillips, 2000). By three years of age, a baby’s brain has reached almost 90% of its adult size. The growth in each region of the brain largely depends on receiving stimulation, which spurs activity in that region. This stimulation provides the foundation for learning.
Brain Development in Adolescence
Brain imaging technologies have provided evidence of the brain continuing to grow and develop into young adulthood – with current research pointing to the brain continuing grow and develop into the thirties (Lebel & Beaulieu, 2011). Right before puberty, adolescent brains experience a growth spurt that occurs mainly in the frontal lobe, which is the area that governs planning, impulse control, and reasoning. During the teenage years, the brain goes through a process of pruning synapses—somewhat like the infant and toddler brain— and also sees an increase in white matter and changes to neurotransmitter systems (Konrad, Firk, & Uhlhaas, 2013).
As the teenager grows into a young adult, the brain develops more myelin to insulate the nerve fibres and speed neural processing, and this myelination occurs last in the frontal lobe. Brain imaging comparisons between the brains of teenagers and the brains of young adults have shown that most of the brain areas were the same—that is, the teenage brain had reached maturity in the areas that govern such abilities as speech and sensory capabilities. The major difference was the immaturity of the teenage brain in the frontal lobe and in the myelination of that area (National Institute of Mental Health, 2001). Normal puberty and adolescence lead to the maturation of a physical body, but the brain lags behind in development, especially in the areas that allow teenagers to reason and think logically. Most teenagers act impulsively at times, using a lower area of their brains—their ‘gut reaction’—because their frontal lobes are not yet mature. Impulsive behaviour, poor decisions, and increased risk-taking are all part of the normal teenage experience. Given the dynamic nature of brain development in the teenage years, decision making during this developmental stage appears to be heavily influenced by heightened emotions, with limitations in the capacity for cognitive processes linked to the prefrontal cortex (Chamberlain, 2009).
Plasticity—The Influence of Environment
Researchers have coined the term ‘plasticity’ to describe the changes in the brain in response to changes in the environment, and its related patterns of activation in the brain (Perry, 2006). The rate and types of changes in neuronal pathways has been found to be different in each stage of development (Perry, 2006). For example, the lower parts of the brain – controlling basic functions such as breathing and heart rate – have been found to be less flexible, or plastic, than the higher functioning cortex, that control higher level cognitions. While the plasticity of the areas of brain like prefrontal cortex decreases as a child gets older, some degree of plasticity remains in the brain (Perry, 2006). Research continues to inform our understanding of lifelong learning, coping and resilience in the face of adverse events. The developing brain’s ongoing adaptations are the result of both genetics and experience. Our brains prepare us to expect certain experiences by forming the pathways needed to respond to those experiences. For example, our brains are ‘wired’ to respond to the sound of speech; when babies hear people speaking, the neural systems in their brains responsible for speech and language receive the necessary stimulation to organise and function (Perry, 2006).
The activation of appropriate pathways in development has been found to be important. For example, research has found that the more babies are exposed to language in the form of caregivers and adults speaking, the stronger their neural pathways related to speech and language. This principle of neurodevelopment has been referred to as ‘use it or lose it’ (Shore, 2015) where through processes of creating, strengthening, and discarding synapses that our brains adapt to the challenges and opportunities of their developmental environment. Regardless, all children need stimulation and nurturance for healthy development. When these are lacking in cases of maltreatment or deprivation, the child’s brain development may be impacted. The brain’s plasticity to its environment means that it changes in response to a negative environment just as readily as it will adapt to a positive one.
Regardless of the general environment, though, all children need stimulation and nurturance for healthy development. If these are lacking (e.g., if a child’s caretakers are indifferent, hostile, depressed, or cognitively impaired), the child’s brain development may be impaired. Because the brain adapts to its environment, it will adapt to a negative environment just as readily as it will adapt to a positive one.
Current research on brain development suggests that there are sensitive periods for development of certain capabilities. These are referred to as ‘windows of proximal development’ – a time in the developmental process when certain parts of the brain may be most susceptible to particular experiences (Siegal, 2015). Animal studies have shed light on sensitive periods, showing, for example, that animals that are artificially blinded during the sensitive period for developing vision may never develop the capability to see, even if the blinding mechanism is later removed.
It is more difficult to study human sensitive periods, but we know that, if certain synapses and neuronal pathways are not repeatedly activated, they may be discarded, and their capabilities may diminish. For example, infants have a genetic predisposition to form strong attachments to their primary caregivers, but they may not be able to achieve strong attachments, or trusting, durable bonds if they are in a severely neglectful situation with little one-on-one caregiver contact. Children from Romanian institutions who had been severely neglected had a much better attachment response if they were placed in foster care—and thus received more stable parenting—before they were 24 months old (Smyke, Zeanah, Fox, Nelson, & Guthrie, 2010). This indicates that there is a sensitive period for attachment, but it is likely that there is a general sensitive period rather than a true cut-off point for recovery (Zeanah, Gunnar, McCall, Kreppner, & Fox, 2011).
While sensitive periods exist for development and learning, we also know that the plasticity of the brain often allows children to recover from missing certain experiences. Both children and adults may be able to make up for missed experiences later in life, but it is likely to be more difficult. This is especially true if a young child was deprived of certain stimulation, which resulted in the pruning of synapses (neuronal connections) relevant to that stimulation and the loss of neuronal pathways. As children progress through each developmental stage, they will learn and master each step more easily if their brains have built an efficient network of pathways to support optimal functioning.
The organising framework for children’s development is based on the creation of memories. When repeated experiences strengthen a neuronal pathway, the pathway becomes encoded, and it eventually becomes a memory. Children learn to put one foot in front of the other to walk. They learn words to express themselves. And they learn that a smile usually brings a smile in return. At
some point, they no longer have to think much about these processes—their brains manage these experiences with little effort because the memories that have been created allow for a smooth, efficient flow of information.
The creation of memories is part of our adaptation to our environment. Our brains attempt to understand the world around us and fashion our interactions with that world in a way that promotes our survival and, hopefully, our growth, but if the early environment is abusive or neglectful, our brains may create memories of these experiences that adversely colour our view of the world throughout our life.
Babies are born with the capacity for implicit memory, which means that they can perceive their environment and recall it in certain unconscious ways (Applegate & Shapiro, 2005). For instance, they recognise their mother’s voice from an unconscious memory. These early implicit memories may have a significant impact on a child’s subsequent attachment relationships. In contrast, explicit memory, which develops around age two, refers to conscious memories and is tied to language development. Explicit memory allows children to talk about themselves in the past and future or in different places or circumstances through the process of conscious recollection (Applegate & Shapiro, 2005).
Sometimes, children who have been abused or suffered other trauma may not retain or be able to access explicit memories of their experiences. However, they may retain implicit memories of the physical or emotional sensations, and these implicit memories may produce flashbacks, nightmares, or other uncontrollable reactions (Applegate & Shapiro, 2005). This may be the case with young children or infants who suffer abuse or neglect.
Responding to Stress
We all experience different types of stress throughout our lives. The type of stress and the timing of that stress determine whether and how there is an impact on the brain. The National Scientific Council on the Developing Child (2014) outlines three classifications of stress:
- Positive stress is moderate, brief, and generally a normal part of life (e.g., entering a new child care setting). Learning to adjust to this type of stress is an essential component of healthy development.
- Tolerable stress includes events that have the potential to alter the developing brain negatively, but which occur infrequently and give the brain time to recover (e.g., the death of a loved one).
- Toxic stress includes strong, frequent, and prolonged activation of the body’s stress response system (e.g., chronic neglect).
Healthy responses to typical life stressors (i.e., positive and tolerable stress events) are complex and may change depending on individual and environmental characteristics, such as genetics, the presence of a sensitive and responsive caregiver, and past experiences. A healthy stress response involves a variety of hormone and neurochemical systems throughout the body, including the sympathetic-adrenomedullary (SAM) system, which produces adrenaline, and the hypothalamic pituitary-adrenocortical (HPA) system, which produces cortisol (National Council on the Developing Child, 2014). Increases in adrenaline help the body engage energy stores and alter blood flow. Increases in cortisol also help the body engage energy stores and also can enhance certain types of memory and activate immune responses. In a healthy stress response, the hormonal levels will return to normal after the stressful experience has passed.
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